Recently, an apparatus for reproducing an optical disc containing digitally recorded image compressed video signals, digital information handled in personal computers, and other digitally stored information (hereinafter referred to as DVD (Digital Video Disc)) was proposed and is approaching commercial application. DVD is capable of recording movies exceeding two hours on a single surface of a disc having the same 12 cm diameter as a high density compact disc. To accurately reproduce information from such a recording medium, an accurate and precise tracking servo is needed. For accurate and precise tracking at the level that DVD requires, it becomes important to generate an extremely precise tracking error signal, one that is even more precise than that used in tracking conventional CD's.
The DVD standard explains that a tracking error signal can be determined from a laser beam spot reflected from the disk. The reflected light spot is detected with a 4-division photo detector. By adding together the outputs obtained from diagonally located divisions in the photo detector, a tracking error can be determined according to a phase difference between those signals. This is based on the principle that a phase difference is produced between the added signals in the diagonal divisions by a tracking error in a phase difference tracking error generation process.
A general phase difference tracking error servo system is shown in FIG. 6. A 4-division photo detector 101 is attached to an optical pickup. The outputs in the diagonally located divisions of the photo detector 101 are added by adders 102 and 103. These signals are shaped into pulses by waveform shapers 106 and 107, compared with each other by a phase comparator 108 and converted into signals corresponding to a phase difference. However, in the case of signals having an extremely high recording density such as in DVD, input signals are subject to the effects of interference from adjacent tracks and various internal and external noises. If so affected, the input signals are not shaped into uniformly shaped square waves between the outputs in the comparators 106 and 107, and a large amplitude noise impulse may be present immediately before the pulses or the pulses may become a lower amplitude wave on one side only. In particular, a phase comparator 108 at the later stage generates a trapezoidal wave phase difference characteristic by reversing the output polarity and by digitally discriminating the advance/delay between two input pulses.
Accordingly, a large output error may be generated by misreading the phase relationship between the pulses if there are abnormal pulses as described above. If such abnormal pulses are generated frequently, accurate tracking operation cannot be expected. Most abnormal operations are caused by pulses having a narrow width, such as the noise impulses described above. To eliminate these narrow pulses, low-pass filters 104 and 105 are inserted before the comparators 106 and 107, respectively.
The output of the phase comparator 108 is a phase error signal which passes through a buffer 109. After the phase error signal is processed in a control circuit 110, the phase error signal controls a driving circuit 111. This driving circuit 111 controls the tracking to the center by moving the spot position of the laser beam forward with respect to the center of the photo detector 101. Thus, the tracking servo signal provides closed loop control to make the phase error zero.
However, a DVD is sensitive to fluctuations in the positional relationship and the sensitivity between the segments of the 4-division photo detector because a DVD has a much higher recording density than a conventional optical disc such as a CD, etc. Further, a DVD is affected delicately by the shapes of pits, lands, and grooves. These physical fluctuation factors appear as a phase error of a pickup output signal for the tracking servo. This phase error is caused by a circuit imbalance and directly causes a tracking error, deteriorating the tracking performance. This circuit imbalance needs to be adjusted to make the circuits equal and to correct the phase error. Thus, in a phase difference type tracking servo, careful advance calibration is needed to eliminate this tracking error.
For tracking optical discs, such as conventional CD's, using the phase difference type tracking servo illustrated in FIG. 6, variable resistors were once used for constructing the low-pass filters 104 and 105. By making the low-pass filters 104 and 105 unbalanced by changing the resistance of either one of these resistors, an originally retained phase difference offset is canceled by a phase offset generated between the low-pass filters 104 and 105.
However, such a method cannot generate a signal having a sufficient amplitude to reverse the comparator in a high frequency region because the amplitude characteristic begins to attenuate before a phase difference offset required for compensation is obtained. The phase difference tracking error signal uses the whole region of the RF signal. Therefore, if the amplitude characteristic is attenuated in the high frequency band, an accurate error signal cannot be generated. The cut-off frequency of a low-pass filter could be set at a high level to limit the regulating range and avoid affecting the amplitude characteristic; however, by doing so, the phase difference offset required for compensation would not be obtained and the noise impulses would not be eliminated.
Although this method was sufficient for tracking CD's having low recording density, the recording density of DVD's is too high for conventional tracking methods to be effective. For DVD tracking, a wide band frequency characteristic is required to generate an accurate phase difference error. Further, a wider phase difference regulating range and more reliable noise impulse elimination is required for DVD tracking due to the higher recording density. However, there is no conventional circuit which can provide the combination of a wide band frequency characteristic, a wide phase difference regulating range, and reliable noise impulse elimination without compromising the performance of at least one of these three objectives.
As described above, it was not feasible to generate a tracking error signal for DVD using a conventional phase difference tracking error generating circuit due to DVD's high recording density. Conventional tracking error generating circuits were unable to regulate sufficiently the phase difference offset and eliminate noise impulses with certainty while maintaining the frequency characteristic necessary for accurate phase error generation in a DVD tracking system.